SUMMARYThe gross form of the nuclear genome varies greatly among plant species in both anatomy and genetic organization. Chromosome number (n) ranges from 2 to over 600, and ploidy from 1 to over 20. The amount of DNA in the unreplicated haplophase genome (the lC value) differs by more than 2500-fold among angiosperms. Although it has been questioned since the 1930s whether such variation is of adaptive significance and whether it is related, perhaps causally, with environmental factors, no direct or causal links have yet been found. However, variation in DNA C-value has far-reaching biological consequences and can be of considerable adaptive and hence ecological significance. Strikingly precise interspecific relationships exist between DNA C-value and many diverse phenotypic characters at the cellular level, and DNA can affect the phenotype in two ways, firstly by expression of its genie content and, secondly, by the biophysical effects of its mass and volume, the latter defined as nucleotypic effects. Nucleotypic variation in DNA C-value sets absolute limits to both the minimum size and mass of the basic unit of plant anatomy (i.e. the cell) and the minumuni time needed to produce a similar cell with newly synthesized organic molecules. Moreover, in complex multicellular vascular plants, such effects at successive cell cycles are additive, so that DNA C-value influences many characters, including growth rate, seed weight, minimum generation time and type of life-cycle. Thus, the nucleotype profoundly affects where, when and how plants grow. Selection for a particular genomic form acting on its spatial or temporal consequences may occur at various levels ranging from the cell to the whole organism and may operate throughout the life-cycle or at just one stage. DNA C-value is often indirectly related to environmental factors which determine time-limited environments via selection acting on the temporal phenotypic consequences of nucleotypic variation. However, in the case of radio-sensitivity, selection for a low DNA C-value may act directly on the nucleotype itself, as the size of the nuclear DNA target directly affects the ability of the plant to survive.Key words: Plant genomic form, DNA C-value, DNA amount, cytoecology, nucleotype.
GENOMIC FORM -CONSTANCY AND VARIATIONMany great discoveries in biology this century have concerned the genome and its form -that the genetic material is DNA, that this has a double helical structure, that this form is important in semi-conservative replication and cell heredity, and that the genetic code is written as triplets of nucleotides. In these important respects, genomic form is the same in all eukaryotes. These discoveries emphasized aspects of constancy in genomic form, except for the nucleotide base sequences of genie DNA whose variation determines the phenotype. However, much cytological work also showed that the genome can vary greatly in both anatomy and genetic organization among taxa.This review is concerned with variation in aspects of gross genomic form in nuclei...